1. Field of the Invention
This invention relates to cross-linked polymers, in particular in the form of insulation on a wire or other conductor.
2Introduction to the Invention
It is known to prepare shaped articles of cross-linked polymers by shaping a composition containing a cross-linking agent, and then subjecting the shaped article to conditions which effect cross-linking. The cross-linking agent can be a so-called chemical cross-linking agent which, when heated, decomposes into active species which effect cross-linking. Alternatively the cross-linking agent can be a radiation cross-linking agent (sometimes called a "pro-rad") which promotes cross-linking when the polymer is irradiated, preferably by electrons, e.g. from an electron beam. One particularly valuable use of cross-linked polymeric compositions is as electrical insulation on a wire or other conductor. Known insulated wires include wires coated with a layer of a radiation crosslinked fluorocarbon polymer, particularly an ethylene/tetrafluoroethylene copolymer (often referred to as an ETFE polymer), which are extensively used for the wiring in aircraft. Military Specification No. MIL-W-22759 sets various standards for such insulated wires. Reference may be made for example to U.S. Pat. Nos. 3,763,222, 3,840,619, 3,894,118, 3,911,192, 3,970,770, 3,985,716, 3,995,091, 4,031,167, 4,155,823 and 4,353,961, the disclosures of which are incorporated herein by reference.